This invention relates to an organic electroluminescent device (organic EL device) wherein an organic layer having a luminescent or light-emitting region is provided between an anode and a cathode.
Lightweight, highly efficient flat panel displays have been extensively studied and developed, for example, for picture display of computers and television sets.
Since cathode-ray tubes (CRT) are high in luminance and exhibit good color reproducibility, they are most widely employed for display at present. Nevertheless, the problems are involved in that the tubes are bulky, heavy and high in power consumption.
For lightweight, highly efficient flat panel displays, there have now been put on the market liquid crystal displays of the active matrix drive type. However, liquid crystal displays have the problems that their angle of field is narrow, they do not rely on spontaneous light and thus, need great power consumption for back light when placed in a dark environment, and they do not have a satisfactory response to high-speed video signals of high fineness which have been expected as being in practical use in future. Especially, a difficulty is involved in making a liquid crystal display with a large picture plane size, along with a problem on its high production costs.
As a substitute therefor, a display of the type using light-emitting diodes may be possible, but such a display is also high in production costs, coupled with another problem that it is difficult to form a matrix structure of light-emitting diodes on one substrate. Thus, when considered as a candidate for a low-cost display used in place of CRTs, this type of display has a great problem to solve before putting to practical use.
As a flat panel display which has the possibility of solving these problems, attention has been recently paid to organic electroluminescent devices (organic EL devices) using organic luminescent materials. More particularly, when using organic compounds as a luminescent material, it has been expected to realize a flat panel display, which makes use of spontaneous light, has a high response speed and has no dependence on an angle of field.
The organic electroluminescent device is so arranged that an organic thin film, which contains a luminescent material capable of emitting light through charge of an electric current, is formed between an optically transparent anode and a metallic cathode. In the research report published in Applied Physics Letters, Vol. 51, No. 12, pp. 913-915 (1987), C. W. Tang and S. A. VanSlyke set forth a device structure (an organic EL device having a single hetero structure), which has a double-layered structure including, as organic thin films, a thin film composed of a hole transport material and a thin film composed of an electron transport material. In the device, luminescence occurs by re-combination of holes and electrons injected from the respective electrodes into the organic films.
In this device structure, either of the hole transport material or the electron transport material serves also as a luminescent material. Luminescence takes place in a wavelength band corresponding to the energy gap between the ground state and the energized state of the luminescent material. When using such a double-layered structure, a drive voltage can be remarkably reduced, along with an improved luminescent efficiency.
Since then, there has been developed a three-layered structure (organic EL device having a double hetero structure) of a hole transport material, a luminescent material and an electron transport material as set out in the research report of C. Adachi, S. Tokita, T. Tsutsui and S. Saito, published in Japanese Journal of Applied Physics, Vol. 27, No. 2, pp. L269-L271 (1988). Moreover, a device structure comprising a luminescent material present in an electron transport material has been developed as set out in the research report of C. W. Tang, S. A. VanSlyke and C. H. Chen published in Journal of Applied Physics, Vol. 65, No. 9, pp. 3610-3616 (1989). Through these researches, evidence has been given to the possibility of light emission of a high luminance at low voltage, thus leading to recent, very extensive studies and developments.
Organic compounds used as a luminescent material are considered to be advantageous in that because of their diversity in kind, a luminescent color can be arbitrarily changed theoretically by changing their molecular structure. Accordingly, it may be easier in comparison with thin film EL devices using inorganic materials to provide, via proper molecular design, three colors of R (red), G (green) and B (blue) having good color purities necessary for full color displays.
However, organic electroluminescent devices still have problems to solve. More particularly, a difficult is involved in the development of a stable red luminescent device with high luminance. In an instance of red luminescence attained by doping DCM [4-dicyanomethylene-6-(p-dimethylaminostyryl)-2-methyl-4H-pyran] in tris(8-quinolinol)aluminium (hereinafter abbreviated as Alq3) for use as a currently reported electron transport material, this is not satisfactory as a display material with respect to both maximum luminance and reliability.
BSB-BCN, which was reported by T. Tsutsui and D. U. Kim in the meeting of Inorganic and Organic Electroluminescence (at Berlin, 1996), is able to realize a luminance as high as 1,000 cd/m2 or over, but is not always perfect with respect to the chromaticity for use as a red color for full color display.
It is now demanded how to realize a red luminescent device which is high in luminance, stable and high in color purity.
In Japanese Patent Laid-open No. Hei 7-188649 (Japanese Patent Application No. Hei 6-148798), it has been proposed to use a specific type of distyryl compound as an organic electroluminescent material. However, the intended luminescent color is blue, not red.
An object of the invention is to provide an organic electroluminescent device, which ensures a high luminance and stable red luminescence or red light emission.
Intensive studies have been made in order to solve the above-stated problems of the prior art, and as a result, it has been found that when using a specific type of styryl compound as a luminescent material, there can be provided a highly reliable red luminescent device, which is very useful for realizing a stable full color display of high luminance.
More particularly, there is provided, according to the invention, an organic electroluminescent device of the type which comprises an organic layer which has a luminescent region and is provided between an anode and a cathode and which contains, as an essential component, an organic material capable of generating luminescence by application of an electric current, wherein the organic layer contains, as an organic luminescent material, at least one styryl compound having an asymmetric structure and represented by the following general formula (1) 
wherein R1 and R2 may be the same or different and, respectively, represent an aryl group of the following general formula (2), (3) or (4): 
wherein R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13, R14, R15, R16, R17, R18, R19, R20 and R21 may be the same or different and independently represent a hydrogen atom, a saturated or unsaturated alkoxyl group preferably having from 1 to 24 carbon atoms, more preferably from 1 to 10 carbon atoms, an alkyl group preferably having from 1 to 24 carbon atoms, more preferably from 1 to 10 carbon atoms, an amino group, an alkylamino group preferably having from 1 to 24 carbon atoms, more preferably from 1 to 10 carbon atoms, or an aryl group, and X represents a substituted or unsubstituted aryl or cyclic hydrocarbon group.
The use, as a luminescent material, of the styryl compound of the above general formula (1) enables one not only to obtain stable red luminescence of high luminance, but also to provide a device which is excellent in electric, thermal or chemical stability. The styryl compounds of the general formula (1) may be used singly or in combination.